1. Jae K. (Jim) Park Dept. of Civil and Environmental Engineering University of Wisconsin-Madison 1

2.  Gas Migration Pathways: pressure gradients and diffusion processes  Methane gas > 5~15% by vol.: explosive  Create vegetation stress and odor problems  Toxicity: H 2 S > 20 ppm; benzene and toluene  Corrosion: acidic condition Refuse Thick cover will impede gas release and increase gas pressure Barrier: pavement, frost, saturated soil Barrier: clay strata Permeable strata Frost cover Gravel lens Trenches and conduits Water table generally acts as a barrier 2

3.  Resource Conservation Recovery Act (RCRA) (1980)  The combustible gas concentration shall not exceed 5% at or beyond the property boundary.  Natural barriers: moist fine-grained soils and saturated coarse-grained soils  Constructed barriers such as trenches, membranes, wells, and vents:  Passive systems: barriers and vents or refuse and adjacent media with large differences in conductivity  Active systems: induce a vacuum to control the flow of gas; preferred when the refuse age is 10 m, and development to be protected is < 1.5 km away  Selection: site specific - economics, degree of protection required, and system reliability 3

4. 4 Approximate zone of influence Condensate trap Gas monitoring probes Landfill boundary Gas header Blower station Gas headerGas well Landfill gas extraction well

5. 5 Perforated gas collection pipe Gas extraction trench Gas monitoring probes Gas collection pipe Condensate trap Blower station Backfilled trench Synthetic membrane liner Landfill boundary

6.  LFG: saturated with water vapor  Temperature in the gas collection pipe system drops 6 LFG containing water vapor Droplets formed Temp. Condensate Pressure buildup Blocking of flow Needs condensate trap

7. 7 Environmental regulations prohibit the return of condensate to the landfill. Condensate

8. 8 Recommended

9.  Based on hazardous characteristics tests for ignitability and TCLP toxicity, LFG condensate and particularly the organic phase may be a hazardous waste.  Some priority pollutants were detected in the organic phase that exceed the proposed regulatory limits. 9 Organic phase (float) (< 1~5% by vol.) Aqueous phase Hydrocarbons, other priority organics, trace moisture Water and trace organics EPA 1988: http://www.p2pays.org/ref/24/23764.pdfhttp://www.p2pays.org/ref/24/23764.pdf

10. UnpublishedDomesticTypical Parameterdata EPA (1988)sewageleachate mg/Lmg/Lmg/Lmg/L BOD1,000~31,2504,000~17,500100~5001,050~32,400 COD476~14,7201,042~30,500250~1,000800~50,700 TOC575~4,90094~23,500100~300700~68,700 10 EPA 1988: http://www.p2pays.org/ref/24/23764.pdfhttp://www.p2pays.org/ref/24/23764.pdf Comparison of BOD, COD, and TOC values for condensate, domestic sewage, and landfill leachate

11. 11

12. 12 Landfill Impermeable liner system Leachate collection system Gravel packed well Blower station Impermeable cover system Landfill gas flared or converted to energy

13. 13

14.  Moisture content: moisture content   permeability and diffusion coefficient  K g = f(  ) and K w = f(  ) where:n =  +  = total porosity; K g = effective K for the gas mixture (cm/sec); and K w = effective K for the soil moisture (cm/sec).  Absolute viscosity: temp.   viscosity  for low density gases Temp. adjustment: Soil condition adj.: D d o = unobstructed diffusion coefficient (cm 2 /sec); and D d = effective diffusion coefficient (cm 2 /sec). (Millington and Quirk, 1961) (Farmer et al., 1980) 14

15.  Methane: colorless, odorless gas, less dense than air, combustible when > 5% by vol.  CO 2 : denser than air, not combustible, acidic conditions  release of alkaline earth metals from soil materials and production of a hardness halo downgradient from the landfill site CH 4 CO 2 Landfill gasAir Viscosity (  10 -5 pa  s)1.031.391.211.71 Mass density,  (kg/m 3 )0.721.971.351.29 Molecular mass, M (g)16.044.030.028.9 Diffusion coefficient in air, D’ (m 2 /sec)1.57  10 -5 Note: Landfill gas defined as 50% CH 4 and 50% CO 2 ; all properties reported at 1 atm and 0  C. 15

16.  Affect gas migration pathways as well as gas velocity and spreading  Grain size, stratification, and spatial variations  Crack or fissure formation, soil pore water content, and intrusion such as an excavation  Hydraulic conductivity of the gas, K g (m/sec) where  = weight density of the gas (N/m 3 ) =  g;  = absolute viscosity (Pa  sec); k s = intrinsic permeability (m 2 ); and g = gravitational constant (m/sec 2 ).  k s : vary by 10 12 between clay and gravel - needs accurate assessment of soil conditions  Pressure: < 12 in. of water (3 kPa) - incompressible, Darcy’s law to calculate gas flow 16

17.  400~1600 gal. of condensable liquids per million ft 3 of gas  water and various hydrocarbons (dichloromethane, dichloroethane, trichloroethylene, toluene, acetone, xylenes, vinyl chloride, and benzene)  Young and Parker (1983) - no significant hazard due to the trace gases from domestic wastes  Odors are worst during the first year after deposition, and organosulfurs and esters play particularly important roles.  Collection and ultimate flaring of landfill gas - min. combustion temp. of 1,500  F and a residence time of 0.3 to 0.5 sec. 17

18.  Very large quantities of air are introduced into the landfill in a localized area, either through natural occurrence or overly aggressive operation of the LFG system  a poorly supported subsurface combustion  CO detected and landfill temp. increase to 400~500°F.  Result from short-circuiting air intrusion into:  Landfill/cover soil interface  Cracks, breaks or imperfections in the cover/cap  Breaks in buried collection piping and extraction wells  Backfill surrounding collection system components (e.g., from the filter or gravel pack of an extraction well or the gravel backfill around a sump)  Subsurface fires: difficult to control or extinguish once initiated 18

19.  Monitor carbon monoxide and temperature for testing potential presence of a landfill fire.  CO: byproduct of incomplete combustion - good indicator  Measurement Diffusion tubes, gas analyzers, gas chromatographs, or aerial thermal imaging  Temperatures within the landfill should be stabilized at or below 140°F and maintained as low as possible for as long as possible. 19

20.  Starve the fire of oxygen - seal all sources of air intrusion - reduce the rate of extraction of LFG or shut down the LFG control system in and adjacent to the affected mass.  Carefully use heavy equipment in areas affected by subsurface fires to avoid a very dangerous hazard that may result from sudden, rapid subsidence (cave-in).  In extreme cases, supply CO 2 to lower temperatures in the area of the fire and thus arresting the chemical pyrolysis that sustains the fire. Applicable for only small fires.  Applying water to subsurface landfill fires is not recommended. Ineffective against CH 4 gas fires. Use along with a surfactant or ‘wet water’ (fire fighting trade). Never attempt to fight an above ground methane fire with water.  Consider landfill fire as a special emergency situation 20